This invention relates to a process for the polymerisation of conjugated diolefins with catalysts based on vanadium compounds in the presence of aromatic vinyl compounds.
It has long been known to polymerise conjugated dienes in the presence of a solvent and such polymerisation has been described, for example, by W. Hoffmann, Rubber to Technology Handbook, Hanser Publishers (Carl Hanser Vertag), Munich, Vienna, New York, 1989. Polybutadiene, for example, is accordingly now predominantly produced by solution polymerisation using coordination catalysts of the Zielger/Natta type, for example based on titanium, cobalt, nickel and neodymium compounds, or in the presence of alkyllithium compounds. The solvent used in each case is highly dependent upon the type of catalyst used. Benzene or toluene as well as aliphatic or cycloaliphatic hydrocarbons are preferably used.
A disadvantage of currently performed polymerisation processes for the production of polydiolefins, such as for example BR, IR, SBR, is the elaborate working up of the polymer solution to isolate the polymers, for example by steam stripping or direct evaporation. A further disadvantage, especially if the polymerised diolefins are to be further processed as impact modifiers for plastics applications, is that the resultant polymeric diolefins must initially be redissolved in a new solvent, for example styrene, so that they may be further processed to yield, for example, acrylonitrile/butadiene/styrene copolymer (ABS) or high impact polystyrene (HIPS).
U.S. Pat. No. 3,299,178 claims a catalyst system based on TiCl4/iodine/Al(iso-Bu)3 for the polymerisation of butadiene in styrene to form homogeneous polybutadiene. Harwart et al., Plaste und Kautschuk, 24/8 (1977) 540, describe the copolymerisation of butadiene and styrene using the same catalyst system and the suitability of the catalyst for the production of polystyrene.
It is known from U.S. Pat. No. 4,311,819 to use anionic initiators for the polymerisation of butadiene in styrene. The disadvantage of the anionic initiators is that they result in the formation of butadiene/styrene copolymers (SBR) which, in relation to the butadiene units, permit only slight control of microstructure. It is only possible to increase the proportion of 1,2 or 1,4-trans units by adding modifiers, which results in an increase in the glass transition temperature of the polymer. Using anionic initiators, it is not possible to produce an SBR having an elevated cis content in which the 1,4-cis content, relative to the butadiene content, is above 40%, particularly preferably above 60%. This fact is primarily disadvantageous because SBR is formed in this process in which, in comparison with homopolymeric polybutadiene (BR), a rising styrene content results in a further increase in the glass transition temperature. However, if the rubber is to be used for impact modification of for example HIPS or ABS, an elevated glass transition temperature of the rubber has a disadvantageous effect on the low temperature properties of the material.
Kobayashi et al, J. Polym. Sci., Part A, Polym. Chem., 33 (1995) 2175 and 36 (1998) 241 have described a catalyst system consisting of halogenated rare earth acetates, such as Nd(OCOCCl3)3 or Gd(OCOCF3)3, with tri(isobutyl)aluminium and diethyl-aluminium chloride, which allows the copolymerisation of butadiene and styrene in the inert solvent hexane. Apart from the presence of inert solvents, the disadvantage of these catalysts is that, at a styrene incorporation of as little as approx. 5 mol. %, the catalyst activity falls to below 10 g of polymer/mmol. of catalyst/h and that the 1,4-cis content of the polymer falls distinctly as the styrene content rises.
U.S. Pat. No. 5,096,970 and EP 304088 describe a process for the production of polybutadiene in styrene using catalysts based on neodymium phosphonates, organic aluminium compounds, such as di(isobutyl)aluminium hydride (DIBAH), and a Lewis acid containing halogen, such as ethylaluminium sesquichloride, in which butadiene is reacted in styrene without further addition of inert solvents to yield a 1,4-cis-polybutadiene.
A disadvantage of this catalyst is that the resultant polymers have a very low content of 1,2 units of below 1%. This is disadvantageous because a higher 1,2 content in the polymer has a favourable effect on the grafting behaviour between rubber and the polymer matrix, for example homo- or copolymers of vinyl aromatic compounds.
The rubber solutions in styrene described in the stated patent publications have been used for the production of HIPS by combining the rubber solutions in styrene with free-radical initiators once the unreacted monomer had been removed.
The object of the present invention was accordingly to provide a process for the polymerisation of conjugated diolefins in vinyl aromatic solvents, by means of which it is possible to obtain polydiene rubbers having an elevated proportion of double bonds in cis position of above 50% and a 1,2 unit content of 10% to 30%. It should moreover be possible to achieve an elevated conversion of the conjugated diolefins used of above 50%, wherein only less than 1% of the introduced vinyl aromatic solvents are reacted.